Semiconductor thin film, semiconductor device and manufacturing method thereof

ABSTRACT

A semiconductor device includes a substrate having an insulating film on its surface, and ac active layer made of a semiconductive thin film on the substrate surface. The thin film contains a mono-domain region formed of multiple columnar and/or needle-like crystals parallel to the substrate surface without including crystal boundaries therein, allowing the active layer to consist of the mono-domain region only. The insulating film underlying the active layer has a specific surface configuration of an intended pattern in profile, including projections or recesses. To fabricate the active layer, form a silicon oxide film by sputtering on the substrate. Pattern the silicon oxide film providing the surface configuration. Form an amorphous silicon film by low pressure CVD on the silicon oxide film. Retain in the silicon oxide film and/or the amorphous silicon film certain metallic element for acceleration of silicon film to a crystalline silicon film. Then, perform a second heat treatment in the halogen atmosphere forming on the crystalline silicon film a thermal oxide film containing halogen, whereby the crystalline silicon film alters to a mono-domain region.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to semiconductor devices, andmore particularly to semiconductor devices having a semiconductive thinfilm as its active layer and the manufacturing method thereof. Theinvention also relates to thin film semiconductor transistors with anactive layer made of crystalline silicon films.

2. Description of the Prior Art

In the recent years semiconductor thin-film transistor (TFT) devices arebecoming more widely used in the manufacture of electronic parts orcomponents, particularly reduced-thickness display devices and digitalintegrated circuit (IC) packages, as the speed and cost advantages ofthese devices increase. As such electronics require higher packingdensity, higher speed, and lower power dissipation, TFTs become morecritical in performance and reliability. Some prior known TFTs come witha silicon thin film as formed on a substrate with a dielectric surface,which film may typically measure several tens to several hundredsnanometers (nm) in thickness.

One typical TFT structure includes an active region as defined betweenspaced-apart source and drain regions for selective formation of achannel region therein. The active region, namely, the channel formationregion and its associated source/drain junction regions may play animportant role to determine the performance of TFT as a whole.

This can be said because the resistance of a current path from thesource to drain through the channel—i.e., the mobility of minoritycarriers—can strictly reflect the overall electrical characteristics ofTFTs.

Conventionally, amorphous silicon films have been generally employed asthe semiconductor thin film constituting the active layer of TFTs. Theseamorphous silicon films may be fabricated by plasma chemical vapordeposition (CVD) and low pressure thermal CVD techniques.

Unfortunately, the use of such amorphous films is encountered with aproblem that where TFTs are required to exhibit higher speeds inoperation, amorphous films are incapable of tracing such trend due toits inherently lowered mobility of charge carriers. To this end, siliconthin films with enhanced crystallinity (to be referred to as the“crystalline silicon film” hereinafter) should be required.

One example is that liquid crystal display (LCD) devices of theactive-matrix type or passive type come with peripheral circuitry whichrequires use of driver circuits for driving picture elements or “pixels”each incorporating TFTs, controller circuits for treating image or videosignals to be displayed, data storage circuits for storing several kindsof information, and others.

It is especially required that the controllers and storage circuits beequivalent in performance to integrated circuits (ICs) using knownsingle-crystalline silicon wafers. Accordingly, where these circuits areintegrated by use of thin film semiconductor as formed on a substrate,it is strictly required to fabricate on the substrate a crystallinesilicon film that is identical in crystallinity to thesingle-crystalline materials.

One prior known approach to form such crystalline silicon film on thesubstrate has been disclosed, for example, in Published UnexaminedJapanese Patent Application (PUJPA) No. 6-232059 to be assigned to thepresent assignee. In this prior art a chosen metallic element orelements are employed to facilitate or accelerate crystal growth ofsilicon, which is subject to thermal or heat treatment at a temperatureof 550° C. for four hours. With this, resultant crystalline silicon filmoffers enhanced crystallinity. A similar approach has also beendisclosed in PUJPA No. 6-244103.

Unfortunately, even when the above technique is applied to themanufacture of an active layer of TFTs, resultant TFTs for use inprocessor circuits or memory circuitry remain insufficient incrystallinity. As the semiconductor manufacturers are commerciallydemanded to improve the TFT speed endlessly, employing the above priorart approach to provide the TFT active layer will be unable to catch upthe strict demands due to their inherent limitations as to improvementsof the crystallinity.

Especially, in order to achieve the crystalline silicon film havingexcellent crystallinity identical to that of single-crystallinematerials, it should be required that substantially no crystal grainboundaries be present therein. This is because of the fact that thepresence or inclusion of such crystal boundaries badly behaves as anenergy barrier which can disturb movement or progress of electronsbetween adjacent crystal grains.

The mechanism of crystal growth by use of the above technique will beanalyzed by subdividing the process thereof into four steps inconnection with FIGS. 10A to 10F.

See FIG. 10A. A silicon oxide film 11 is formed on the surface of asubstrate as a buffer layer. An amorphous silicon film 13 is formedoverlying the silicon oxide film 11. Oxide film 11 has a configuration12 on its surface, which has been formed due to the inherent surfaceroughness and/or presence of contaminants. The surface configuration 12is depicted here as local projections for purposes of explanation only.The amorphous silicon film 13 is provided with a few of drops of coatingsolution containing a metallic element(s) for acceleration orfacilitation of crystallization, and then rotated with circularrotational speed sufficient to centrifugally spin the coating solutionuniformly and radially across the upper surface of film 13. A coat layer14 is thus deposited covering the upper surface of amorphous siliconfilm 13 as shown in FIG. 10A. Coat layer 14 may contain nickel (Ni) asretained therein.

The structure of FIG. 10A is heated up to a temperature of from 500 to700° C. for crystallization of amorphous silicon film 13. Thus, themetallic element tends to internally diffuse isotropically withinamorphous silicon film 13 as designated by arrows shown in FIG. 10B,finally reaching the interface between films 11, 13. This is the firstof the prescribed four steps for analysis.

As a result of such internal diffusion, the metallic element migratesthrough the interface between films 11, 13 leading to segregation on thelocal projections of surface configuration 12 as shown in FIG. 10C. Thisis the second step. Such segregation occurs due to the fact that themetallic element attempts in nature to require stable cite of energy—inthis case, the surface projections 12 act as such segregation cite.

At this time the surface projections 12 serving as the segregation citecontain one or several metallic elements at an increased concentrationpermitting occurrence of a crystal nucleus therein. Our study revealsthe fact that where the metallic element is nickel, the crystal nucleustakes place when the concentration thereof is equal to or greater than1×10²⁰ atoms per cubic centimeter. Crystal growth occurs with thecrystal nucleus being as a starting point or “seed.” Such growth beginswith vertical crystallization substantially perpendicular to the siliconfilm surface as shown by numeral 15 in FIG. 10D. This is the third step.

The vertical crystal growth regions 15 of FIG. 10D is such thatcrystallization progresses while pushing upward the highly condensedmetallic element(s) contained therein; accordingly, these elements areforced to reside in the surface of the overlying amorphous silicon film13 at an increased concentration. This results in that vertical growthregions 15 remain greater in concentration of metallic element than theremaining regions of film 13.

Then, as shown in FIG. 10E, crystal growth starts in a specificdirection substantially parallel to the substrate surface (as designatedby arrows shown) with a respective interface 16 between the amorphoussilicon film 13 and the individual vertical growth region 15 being as aseed of crystallization. This results in lateral growth of crystals 17,which is the fourth step. Each lateral crystal 17 may be a mixture ofcolumnar and/or needle-like crystals having the crystal width which issubstantially identical to the thickness of amorphous silicon film 13 asshown in FIG. 10E.

The lateral crystals 17 grow in the direction parallel to the substratesurface in such a manner that laterally growing opposite crystals 17come closer to each other in one amorphous silicon region as partitionedby adjacent vertical crystal growth regions 15. When these oppositelateral grown crystals 17 are in contact with each other at the frontportions thereof, crystal growth is terminated providing a correspondingcrystal growth boundary 18 therebetween as shown in FIG. 10F. Aresultant lateral crystal growth region 19 with such boundary 18exhibits relatively regular or well-aligned crystallinity.

One disadvantage faced with the prior art approach to crystallization isthat the presence or formation of a number of segregation cites on thefilm surface acts to increase the density of crystal nucleus, which inturn undesirably causes the individual crystal grain to disturb thegrowth of neighboring crystals. This should result in a decrease indiameter of crystal grain. In other words, where a crystalline siliconfilm is formed as the TFT active layer by use of the crystal growthscheme as taught by the prior art, the resulting film must containcrystal boundaries therein. This is a serious bar to achievement ofimproved crystallinity which is equivalent to that of single-crystallinesemiconductor materials.

If the occurrence frequency of such crystal nucleus is decreased, thenthe crystal grains will increase in diameter accordingly. However, eventhis is the case, the positional controllability of crystal nucleusremains very difficult or nearly impossible. Generally, the actuallocation of segregation cites may be determined depending upon wherethese cites of metallic element are positioned. With the prior art, thesegregation cites, such as the local surface projections 12 of FIG. 12A,appear on the film surface at random in position. This means that itremains difficult or almost impossible to well control the exactlocation of segregation cites. In addition to this, it is shown by thepresent inventors that any crystalline silicon films fabricated inaccordance with the prior art approach discussed previously must containtherein the metallic element that has been utilized during thecrystallization process, which disadvantageously serves to deterioratethe stability and reproducibility of semiconductor devices employing theresultant semiconductive film as its active layer or the like.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a new andimproved approach that avoid the problems faced with the prior art.

It is another object of the invention to provide an improvedsemiconductor device capable of avoiding the problems faced with theprior art as well as the method for forming the same.

It is a still another object of the invention to provide a semiconductordevice capable of offering enhanced performance and reliability withouthaving to make use of single-crystalline semiconductor wafers.

It is yet another object of the invention to form a mono-domain regionhaving crystallinity as equivalent to single-crystalline materials on acarrier body with a dielectric surface.

It is a further object of the invention to provide a semiconductordevice having an active layer overlying a substrate with a dielectricsurface and being made of a mono-domain region as equivalent incrystallinity to single-crystalline materials.

To attain the foregoing objects, in accordance with one aspect of thepresent invention, a semiconductive thin film is formed on a substratehaving an insulating film on a surface thereof, and is comprised of amono-domain region which includes therein a plurality of columnar and/orneedle-like crystals that are substantially parallel to the substratesurface. The insulating film underlying said thin film has aspecifically defined surface configuration with an intended profilepattern fabricated.

In accordance with another aspect of the invention, a semiconductor thinfilm on a substrate having an insulating film on a surface thereof iscomprised of a mono-domain region which includes a plurality of columnaror needle-like crystals while substantially excluding presence ofcrystal grains therein. The insulating film underlying thesemiconductive thin film has its surface with a specific configurationwith an intended pattern fabricated. The semiconductive thin filmconstituting the mono-domain region contains therein hydrogen andhalogen elements at a carefully determined concentration that is lessthan or equal to five (5) atomic percent (at %). The halogen element isselected from the group consisting of chlorine, bromine and fluorine.

In accordance with a still another aspect of the invention, thesemiconductor device makes use of the mono-domain region exclusively forformation of the active layer thereof. In this case, essentially nocrystal grain boundaries exist within the mono-domain region.

In accordance with a further aspect of the invention, a method offorming a semiconductor thin film on a substrate with an insulating filmon its surface is provided which method includes the steps of forming asilicon oxide film by sputtering techniques on the insulating film ofthe substrate, patterning said silicon oxide film to provide an intendedpattern of surface configuration thereon, forming an amorphous siliconfilm by a low pressure chemical vapor deposition on the silicon oxidefilm, retaining in at least one of the silicon oxide film and theamorphous silicon film a metallic element for facilitation oracceleration of crystallization thereof, performing a first heattreatment to alter in nature the amorphous silicon film to a crystallinesilicon film, performing a second heat treatment in an atmospherecontaining halogen elements thereby forming on the crystalline siliconfilm a thermal oxide film containing halogen elements while causing thecrystalline silicon film to alter to a mono-domain region due to thesecond heat treatment, and removing the thermal oxide film. Themono-domain region thus fabricated is for use as the active layer ofsemiconductor devices.

It should be noted here that the term “mono-domain region” is used torefer to resultant lateral growth crystal region as formed using thesemiconductor thin film manufacturing method of the invention, by takingaccount of the fact that this region is enhanced in crystallinitysufficiently to be regarded as the single crystal in substance. Aprincipal feature of the mono-domain region is that no grain boundariesare found through its entire region, and accordingly, any latticedefects or dislocations are suppressed or eliminated which are otherwiseoccurred due to presence of transitions and interlayer defects. Anotherfeature is that the mono-domain region avoids inclusion of any metallicelements otherwise acting to badly influence the fundamentalcharacteristics of the semiconductor device.

It should also be noted that the absence of crystal grain boundariesalso covers in intended meaning that even if a few grain boundaries arepresent, these remain electrically inactive. As such electrical inactivegrain boundaries, there have been reported the {111} grain boundary,{111} interlayer defect, {221} twin-crystal grain boundary, and {221}twist twin-crystal grain boundary (R. Simokawa and Y. Hayashi, Jpn. J.Appl. Phys., 27 (1987) at pp. 751 to 758).

It is considered by the present inventors that the crystal grainboundaries contained in the mono-domain region remain as electricallyinactive ones at increased possibility. In other words, even where someboundaries may be observed therein, such are electrically inactiveregions which will no longer reflect or disturb the movement of chargecarriers therein. In this sense these are electrically “transparent” tothe flow of internal current.

The fabrication of such mono-domain region in accordance with thepresent invention incorporates a unique concept of precisely controllingthe location of crystal nucleus by increasing the diameter of respectivecrystal grains thereby to decrease in number any possible crystal grainboundaries therein.

The principal approach of the present invention is to extremely smooththe surface of an insulating or dielectric film underlying an amorphoussilicon film. To do this, a buffer layer is provided beneath theamorphous silicon film. The buffer layer may be a silicon oxide film asformed by sputtering techniques using an artificial quartz as a target.The composition table of one recommended artificial quartz target usedis shown in FIG. 18 for reference. The silicon oxide film thus formedexhibits an enhanced density and increased smoothness, which is freefrom presence of any possible segregation cites thereon otherwiseoccurring in the prior art.

Another key concept of the invention is to pattern the silicon oxidefilm forming thereon a specific surface configuration of an intendedprofile pattern, which may include a projection or recess in thesurface. In other words, intentional formation of segregation cites ofcertain metallic element for acceleration or facilitation ofcrystallization makes it possible to well control the actual locationsof crystal nucleus that might take place on the film surface. This mayenable formation of grown crystals of any desired sizes at any desiredlocations, and thus provide great contribution to the semiconductormanufacturers.

The use of low pressure chemical vapor deposition (CVD) techniques toform the amorphous silicon film on the substrate is also an importantfeature of the instant invention. As compared to amorphous silicon filmsformed using plasma CVD techniques, those formed by the low pressure CVDmethod offer advantageous characteristics which follow: small in contentof hydrogen, more dense in film quality, and less in rate of occurrenceof natural crystal nucleus. A decrease in natural nucleus occurrencerate leads to an increase in accuracy of the controllability of crystalnucleus locations.

A further key concept of the invention is that the heat treatments areperformed in a specific atmosphere containing therein halogen elementsthus enabling fabrication of a mono-domain region. This is based on thepresent inventors' experimentation and analysis to search for a suitablemeans for altering the resultant crystals thus grown with relativelylarge-size crystal grains formed therein to single crystals orequivalents thereto (more precisely, altering to mono-domain regions).

These and other objects, features and advantages of the invention willbe apparent from the following more particular description of preferredembodiments of the invention, as illustrated in the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1F and 2A to 2C illustrate, in schematic cross-section, someof the major steps in the formation of a semiconductor thin-film withmono-domain regions in accordance with one preferred embodiment of theinvention.

FIGS. 3A to 3C are diagrammatic representations each for explanation oflateral crystal growth regions in a semiconductor device in accordancewith the principles of the invention.

FIGS. 4A to 4E illustrate, in schematic cross-section, some of the majorsteps in the formation of a semiconductor device having a mono-domainactive layer in accordance with another embodiment of the invention.

FIG. 5 is a graph showing the electrical characteristics of a thin-filmtransistor (TFT).

FIG. 6 is a graph showing the relation of the vapor pressure nickelchloride versus temperature.

FIG. 7 is a graph showing the distribution of concentration of chlorineas contained in a crystalline silicon film.

FIG. 8 is a diagrammatic representation for explanation of problemsfaced with a known semiconductor-on-insulator (SOI) structure.

FIG. 9 shows a planar structure of a mono-domain region in accordancewith a further embodiment of the invention.

FIGS. 10A to 10F illustrate in schematic cross-section some of the majorsteps in the formation of a semiconductor thin-film with crystallinityin accordance with one prior known approach.

FIG. 11 is a perspective view of a liquid crystal display (LCD)substrate having multiple patterned TFT active layers as formed in amono-domain region in accordance with a further embodiment of theinvention.

FIGS. 12A to 12E, 13A-13D and 14A-14B illustrate in schematiccross-section some of the major steps in the formation of asemiconductor device in accordance with a further embodiment of theinvention.

FIGS. 15A to 15D depict in schematic cross-section some of the majorsteps in the formation of a semiconductor device in accordance with afurther embodiment of the invention.

FIGS. 16A and 17A respectively show one cell section of a memory arrayof a dynamic random access memory (DRAM) and that of a static RAM(SRAM), and FIGS. 16B and 17B show cross-sections of each of the cellsof the preceding figures.

FIG. 18 is a table showing the composition ratio of an artificial quartztarget.

FIGS. 19A to 19D illustrate in schematic cross-section some of the majorsteps in the formation of a semiconductor device in accordance with afurther embodiment of the invention.

FIGS. 20A to 20F show several exemplary electronic devices to which thesemiconductor device of the invention is preferably applicable.

DETAILED DESCRIPTION OF THE INVENTION EXAMPLE 1

A fabrication method of a “mono-domain” region which is a key to theinvention will be fully described with reference to FIGS. 1A to 1F. SeeFIG. 1A. This is a diagrammatic depiction (not drawn to scale) of asubstrate 101 having an insulating or dielectric surface. The substrate101 is made of a material of enhanced heat resistance, such as quartz,silicon or the like. As shown, a silicon oxide film 102 is formed bysputtering techniques on substrate 101 with an artificial quartz beingas the target. The silicon oxide film 102 exhibits an increased flatnessand smoothness on its top surface; for example, any possible surfaceconfiguration of film 102 measures 3 nanometers (nm) or less in heightand 10 nm or greater in width. This surface configuration will bedifficult or nearly impossible to be visually recognized even by use ofatomic force microscopy (AFM) measurement schemes.

After formation of the extra flat silicon oxide film 102, resultantstructure is then subjected to patterning process thereby intentionallyforming a minute island pattern of a rectangular or square profile onthe top surface of film 102. The island may be a projection 103 as shownin FIG. 1A. This surface island 103 may alternatively be a recess of acorresponding profile in the surface of film 102. Preferably, island 103is formed such that the height thereof is half the thickness of anoverlying amorphous silicon film 104, which is formed by plasma chemicalvapor deposition (CVD), sputtering or low pressure CVD techniques to apredetermined thickness of 10 to 75 nm; preferably, 15 to 45 nm. Whenlow pressure CVD techniques are employed, disilane (Si₂H₆) or trisilane(Si₃H₈) may be used as the film formation gas. Carefully setting thethickness of film 104 to fall within the prescribed range enablesfabrication of a semiconductor device using a later-formed crystallinesilicon film as its active layer while lowering the turn-off currentthereof.

It should be noted that the amorphous silicon film 104 formed using thelow pressure CVD method remains less in natural nucleus occurrence rateduring a later crystallization step. The term “natural nucleusoccurrence rate” refers to the ratio of crystal nucleus which willpossibly take place by application of thermal or heat energy withoutcausing amorphous silicon film 104 to accept any influence orinterference with certain metallic element such as nickel (Ni) as usedto accelerate or facilitate crystallization. Such reduction in naturalnucleus occurrence rate is desirable for accomplishment of enlargedcrystal grains with an increased diameter. This is as a result of adecrease in mutual interference between respective crystals (that is, areduction in possibility of termination of crystal growth due to contactor “crash” of neighboring crystals that laterally grow to come closer toeach other).

It should also be noted that during formation of the amorphous siliconfilm 104, great care is taken to maintenance of cleanness on the exposedsurface of silicon oxide film 102. As has been discussed in connectionwith the prior art as shown in the introductory part of the description,if contaminants are present on the film surface, these will badly behaveas segregation cites of the crystallization accelerator metal elementsproviding starting points or “seeds” for undesired growth of crystalnucleus.

After formation of the amorphous silicon film 104, resultant structureis then irradiated with ultra violet (UV) rays in the oxygen-gasatmosphere forming an extra thin oxide film (not shown) on film 104.This oxide film is for improving the wetness of a coating solution asemployed to introduce chosen metallic element thereinto. Then, a fewdrops of solution containing therein certain metallic element(s) actingas a crystallization accelerator are provided on the exposed filmsurface at a predefined concentration, forming a liquid film (notshown). The metallic element used here may be iron (Fe), cobalt (Co),nickel (Ni), ruthenium (Ru), rhodium (Rh), palladium (Pd), osmium (Os),iridium (Ir), platinum (Pt), copper (Cu) or gold (Au), or a combinationthereof. Use of Ni may be recommendable in this case since Ni remarkablyexhibits preferable effects for fabrication of an intended mono-domainregion as will be described later in the description. Note here that inview of the possibility of residual impurity during later heattreatments, use of nickel nitrate solution is preferable rather thannickel acetate solution due to the fact that the latter inherentlycontains therein carbon which will undesirably continue by carbonizationto reside within a thermal oxide film to be formed by heat treatment aswill be discussed later.

The structure of FIG. 1A is then rotated with circular rotational speedsufficient to centrifugally spin the coating solution uniformly andradially across the upper surface of amorphous silicon film 104. Anickel coat solution 105 is thus disposed covering the upper surface offilm 104 with the extra thin oxide film (not shown) being laidtherebetween. Very importantly, a surface projection 106 is defined onthe top surface of amorphous silicon film 104. Projection 106 isessentially self-aligned with and substantially identical in profile tothe underlying island 103 positioned just beneath the surface projection106 as embedded between films 102, 104.

During the spin-coating process, due to the presence of surfaceprojection 106 on the top surface of the structure of FIG. 1A, Ni-coatsolution tend to adhere by surface tension to the side walls ofprojection 106 providing a locally increased concentration of nickeltherearound on the surface. This advantageously serves during a latercrystallization step to permit acceleration of intended crystal growth,that is, lateral crystallization in parallel to the substrate surface.

While the illustrative embodiment assumes execution of the spin-coatingof the solution on amorphous silicon film 104, the same mayalternatively be carried out on the silicon oxide buffer layer 102 priorto formation of amorphous silicon film 104. Still alternatively, thecoating of such solution may be performed with respect to both films102, 103 if appropriate.

Then, the structure of FIG. 1A is heated for removal of hydrogen at atemperature of 450° C. in inactive gaseous atmosphere for one hour.Thereafter, the structure is further heated for crystallization ofamorphous silicon film 104 at a predetermined temperature ranging from500 to 700° C., preferably 550 to 600° C., for 4 to 8 hours. This willbe called the “first heat treatment” hereinafter.

The crystallization of amorphous silicon film 104 is as follows. As afirst step, nickel in amorphous silicon film 104 is thermally activatedto isotropically diffuse downward within film 104 as shown by arrows inFIG. 1B.

Then, as a second step, the nickel attempts to migrate at the interfacebetween the silicon oxide buffer layer 102 and amorphous silicon film104 segregating toward island 103. This may function as a segregationcite with the intentionally formed projection profile of island 103 asshown in FIG. 1C.

Next, as shown in FIG. 1D, a crystal nucleus takes place at theperiphery of segregation cite island 103. This occurs when the nickelconcentration becomes equal to or greater than 1×10²⁰ atoms per cubiccentimeter at the periphery of island 103. Creation of such crystalnucleus allows crystal growth or crystallization to progress in aspecific direction substantially normal to the silicon film surface.This is the third step of crystallization. A resultant vertical crystalgrowth region 107 thus formed contains therein nickel at an increasedconcentration as discussed previously.

As a fourth step of crystallization, lateral crystal growth occurs withthe vertical growth region 107 being as a seed of crystal growth in adirection substantially parallel to the silicon film surface. Lateralcrystal growth regions 108 are gradually formed at the opposite sides ofregion 107 in amorphous silicon film 104 as shown in FIG. 1E. Theselateral growth regions 108 are a mixture or combination of a pluralityof columnar and/or needle-like crystal segments which are essentiallyidentical or aligned in direction to one another, thus exhibitingsuperior crystallinity as compared with vertical growth region 107.

During the fourth step, controlling of location of the segregation citebeing formed may enable growth of enlarged crystal grains with increaseddiameter without causing the individual gain to accept unintentionalinfluence or interference with the remaining, adjacent crystal grains.This in turn permits formation of intended crystals of any desired sizeat any desired locations insofar as design parameters are suitablyselected in such a manner that the segregation cite is preciselycontrolled in location. Note here that how the crystal grains areenlarged in size is determined depending upon how long the heattreatment is to be performed at suitably defined temperatures;accordingly, the actual crystal size may be freely designed in light ofthe cost limitation in the manufacture of semiconductor devicesrequired. Determination of crystal grain size may also be done by takinginto consideration the fact that high-temperature heat treatment will beadditionally carried out during a later single-crystallization processwhich also permits further crystal growth.

As a result of the lateral crystal growth of FIG. 1E, a crystallinesilicon film 109 is formed on the silicon oxide buffer layer 102. Here,attention should be paid to the fact that the illustrative fabricationtechnique differs in principal concept from the presently availablegraphoepitaxy technology as follows: The graphoepitaxy intends toprovide regularity to the surface configuration of an underlyingfoundation coat film for alignment in distribution of a crystallinesilicon film by utilizing its inherent nature that the most stablesurface appears at an intended surface location during crystallizationof an overlying amorphous silicon film; in contrast, the fabricationmethod embodying the present invention is featured in varying thesurface energy by changing the surface configuration of a coat filmproviding a specific region which facilitates segregation of thecrystallization accelerator metallic element, nickel here. Consequently,the illustrative embodiment remains distinguishable from thegraphoepitaxy in that the main reason for a change in surfaceconfiguration is to form a crystal nucleus.

A planar view of the crystalline silicon film 109 is depicted in FIG.3A, wherein numeral 301 designates the vertical crystal growth region(107 in FIG. 1D) as formed at the third step of crystallization. In theillustrative embodiment this region 301 exhibits a square shape due toformation of the minute square island pattern. Numeral 302 denotes thelateral crystal growth region (109 in FIG. 1F) as formed at the fourthstep. Lateral growth region 302 has been grown with the centrallylocated vertical growth region 301 being as a seed or nucleus: In thisembodiment, since the seed region 301 can be regarded as a pin point,resultant planar shape resembles a hexagon as a whole as shown in FIG.3A. One possible reason for such shape is as follows. It has been wellknown among those skilled in the art that the crystal growth of siliconwith a nucleus surrounded by the (111) plane results in crystal grainsbeing grown in hexagonal shape. On the other hand, it is shown by thepresent inventors that where nickel is employed as one crystallizationaccelerator metallic element, nickel silicide is formed at the distalend and side portions of each columnar or needle-like crystal duringcrystallization. It is known that this nickel silicide has its stableplane corresponding to the (111) plane. In view of these facts, theplane surrounding vertical growth seed region 301 is mainly constitutedby the (111) plane which is the stable surface of nickel silicide. As aconsequence, it will be seen that the lateral growth region 302 of FIG.3A has a hexagonal shape when crystal growth is done with region 301being as a starter point under the assumption that region 301 is apoint.

As seen from FIG. 3A, the hexagonal lateral growth region 302 may besubdivided into six subregions A to F, each of which can be observed asa single crystal grain. This is because of the fact that dislocationssuch as slip defects take place at certain locations whereat adjacentones of subregions A to F are in contact with each other.

As enlargedly shown in FIG. 3B, one part of lateral growth subregions Ato F consists of a mixture of a plurality of minute columnar orneedle-like crystals. Macroscopically, the high density or “crowd” ofsuch crystals may allow each subregion to look like a single grain as awhole. Note that a respective one of these columnar or needle-likecrystals is a mono-domain region which are free from inclusion orpresence of any crystal grain boundaries therein and thus can be deemedto be a single crystal. Note also that since the individual crystal isgrown while excluding an impurity such as nickel from the insidethereof, metallic silicide is formed on the surface of each crystal,which in turn results in segregation of nickel at crystal boundaries 303in FIG. 3B. Accordingly, the crystal state of FIG. 3B remains as a merecollection of multiple mono-domains: While this state exhibits superiorcrystallinity, it is not attained yet that the individual one ofsubregions A to F of FIG. 3A is filled with a single mono-domain region.

To fully attain the inventive contribution it should be required aspecific process be added for improving the crystallinity of the lateralgrowth region 302 of FIG. 3B. This process will be referred to as the“single-crystallization” hereinafter, and will be described withreference to FIGS. 2A to 2C. Practically, the single-crystallization maybe achieved by heat treatment in the oxidative atmosphere containinghalogen elements, which will be called the “second heat treatment”hereinafter.

The structure of FIG. 1F including the crystalline silicon film 109 issubjected to a further (second) heat treatment so that the exposedcrystalline silicon film 109 is heated at high temperatures which rangefrom 700 to 1100° C. for 1 to 24 hours. Preferably, the film 109 isheated at 800 to 1000° C. for 6 to 12 hours. Very importantly, theatmosphere employed here is designed to contain therein halogen elementsat the step of FIG. 2A. In this embodiment, the second heat treatmentwas performed at a temperature of 950° C. for 6 hours in a chosenatmosphere of oxygen gas that contains therein HCl at concentrationratio (volume density) of 3%. Note here that further inclusion ofnitride gas in the atmosphere may be recommendable for achievement ofsufficient getter effects since it acts to slow the rate of formation ofany oxide films. Note also that while Cl was chosen as the halogenelement in this embodiment with HCl gas being employed as anintroduction material thereof, other kinds of gases may alternatively beused. HF, NF₃, HBr, Cl₂, F₂ and/or Br₂ are examples thereof. Halogenhydrates or organic substances (carbohydrates) are other possibleexamples.

During the second heat process at step of FIG. 2A, the nickel in thecrystalline silicon film 109 heated is gettered due to the chlorine'saction, and thus is removed away as a result of absorption into anoverlying thermal oxide film 110 and/or release toward the atmosphericair. Accordingly, nearly all Ni elements contained are removed from film109 providing a Ni-absent crystalline silicon film 111 covered by thethermal oxide film 110 as shown in FIG. 2B.

The nickel (Ni) removed during the getter step of FIG. 2B has beensegregated as a result of push-out toward crystal boundaries (see 304 inFIG. 3B) during crystallization. It can thus be considered that Ni hasbeen present as nickel silicide at the crystal boundaries. The nickelsilicide is separated as nickel chloride resulting in presence of anumber of unpaired coupling hands of silicon after separation or cutofffrom nickel atoms at grain boundaries therein. Fortunately, severalunpaired coupling hands of silicon atoms are forced during the secondheat treatment at 950° C. to mutually recombine with those of theremaining ones. Still unpaired hands, if any, are filled with those ofhydrogen and halogen elements as also contained in the crystallinesilicon film 111. Due to this, film 111 comes to contain hydrogen andhalogen elements at 5 atomic percent (at %) or less. This ensures thatthe boundaries are in junction with one another with an enhancedmatching property due to such recombination of silicon atoms, enablingexclusion or depletion of any boundaries therein. Furthermore, as aresult of the second heat treatment, transitions, dislocations or anyother possible interlayer crystal defects inside the columnar andneedle-like crystals will disappear almost completely enhancing thecrystallinity of them.

The improvement in crystallinity will be more fully described withreference to FIGS. 3B and 3C for comparison. By heating the crystalstructure of the lateral crystal growth region shown in FIG. 3B, nickelcontained therein is removed away from the film by chlorine's getteraction. When this is done, the coupling or combination between siliconand nickel atoms is cut off forming unpaired coupling hands, which arethen recombined with those of neighboring silicon atoms during the heattreatment to provide a resultant crystal structure as shown in FIG. 3C.

The structure of FIG. 3C comes with several junction interfaces asdesignated by broken lines 304. These interfaces 304 have been formed asa result of the crystal boundaries 303 of FIG. 3B being once dissociateddue to the heat treatment and then recombined thereafter. In each of thecrystal segments A to F of FIG. 3A, the columnar or needle-like crystalstherein are recombined together with superior matching or alignmentproviding the state of FIG. 3C which is nearly perfectly free frominclusion of any crystal boundaries. This ensures that each ofsubregions A to F of FIG. 3A contains no substantive boundaries and noimpurity atoms such as nickel and that each segment behaves as amono-domain region which may eliminate presence of any possible crystaldefects therein Our experimentation using secondary ion massspectrometer (SIMS) analysis revealed the fact that the mono-domainregion was reduced in Ni concentration by one to three orders ofmagnitude.

Turning back to FIG. 2C, after completion of the nickel getter process,the thermal oxide film 110 of FIG. 2B is then removed away, which hasfunctioned as the gettering cite. Removal of film 110 is to preventnickel atoms from re-diffusing into the crystalline silicon film 111. Inthis way there is obtained the crystalline silicon film 111 with areduced nickel concentration as shown. In this region Ni has beenremoved or decreased, by execution of the heat treatment in the halogenatmosphere, down at a target concentration which is low sufficient toensure that residual Ni atoms if any do no longer disturb themanufacture or fabrication of intended semiconductor devices includingTFTs—for example, 1×10¹⁸ atoms per cubic centimeter (atoms/cm³) or less;preferably, 1×10¹⁷ atoms/cm³; more preferably, 1×10¹⁶ atoms/cm³. This inturn leads to an improvement in crystallinity to the extent that themono-domain region can be equivalent in crystal structure tosingle-crystalline materials.

One characterizing feature of the invention is that the mono-domainregion thus formed is for exclusive use as the active layer ofsemiconductor devices, including TFTs.

A semiconductor structure is shown in FIG. 11, which is for use in anactive-matrix liquid crystal display (LCD) device with TFTs each havingan active layer consisting of the mono-domain region of the presentinvention. As shown, the structure includes a substrate 21 having aninsulating or dielectric surface on which an array of patterned TFTactive layers 24 are disposed in rows and columns. Two stripe-shapedelongate surface areas 22 at the opposite side edges of substrate 21 arecertain locations where the vertical crystal growth regions have beenpositioned. A broken line 23 is depicted to show the locus of a linearcrystal grain boundary which has been formed here due to mutual conflictof lateral growth regions. The broken line is used here because suchboundary 23 successfully disappears once after completion of fabricationof patterned active layers 24.

As shown in FIG. 11, the array of active layers 24 is centrally formedon the top surface of substrate 21 while avoiding inclusion of thevertical growth region presence areas 22 and the locus of boundary 23.While the explanation is directed to part of LCD structure illustrated,the above is also true for the remaining ones (not visible in thedrawing) of TFT active layers which will be provided on the order of 10⁶in number on substrate 21.

EXAMPLE 2

A manufacturing method of a TFT structure utilizing the mono-domain ofExample 1 is shown in FIGS. 4A to 4E. While this illustrative embodimentwill be described herein in connection with a top-gate TFT structure,the invention should not exclusively be limited thereto. One skilled inthe art will readily recognize that the method of FIGS. 4A to 4E mayalternatively be applicable to formation of a bottom-gate TFT with thegate electrode being replaced by the one which is high in heatresistance.

As shown in FIG. 4A, a quartz substrate 401 with a silicon oxide film402 and a “pseudo single-crystalline” mono-domain silicon film 403 beinglaminated on the surface of substrate 401 in this order. Note that thefilms 402, 403 may be fabricated using the method shown in FIGS. 1A-1Fand 2A-2C. The silicon film 303 has therein a mono-domain region asdescribed previously. Film 403 is patterned by patterning techniques asshown in FIG. 4A. This patterned film 403 will be later used as theactive layer of a TFT structure.

In the structure of FIG. 4A, another silicon oxide film 404 is depositedby plasma CVD techniques to a predetermined thickness, for example, 150nm. This film 404 will later act as the gate insulating film of TFT.Film 404 may alternatively be made of silicon oxide/nitride or siliconnitride. An aluminum film 405 is then deposited by sputtering to athickness of 500 nm on film 404. Film 405 overlies film 404 and will actas the gate electrode of TFT. Film 405 contains an impurity of scandiumat 0.2 weight percent (wt %). Film 405 may alternatively be made ofother conductive materials, such as tantalum, molybdenum, or others.

The structure of FIG. 4A is then subject to formation of an anode oxidefilm (not shown) of typically 10-nm thick overlying the aluminum film405. This formation process uses as electrolytic solutionethylene-glycol solution as neutralized using ammonia water. Anodeoxidization is carried out in such a way that when the structure is putin the electrolytic solution, film 405 is used as the anode while aplatinum layer (not shown) as cathode therefor. An anode oxide film thusformed at this step is dense enough to improve the contact or adhesioncharacteristics with a later-formed resist mask thereon.

As shown in FIG. 4B, the aluminum film 405 is patterned forming anisland 406 on silicon oxide film 404. The aluminum island 406 will serveas a base layer of the TFT gate electrode. Although omitted from thedepiction of FIG. 4B, a mask layer used for patterning film 405 of FIG.4A is not yet removed and thus continue to reside at this stage.

The structure of FIG. 4B is again subjected to the anode oxidationprocess with island 406 being as the anode. The electrolytic solutionhere may be aqueous solution of 3%-oxalic acid. At this step the anodeoxidization progresses only at the side walls of island 406 due to thepresence of the aforesaid resist mask (not shown). This results information of anode oxide films 407 only at opposite side walls of island406 as shown in FIG. 4C. These side wall films 407 are porous in natureand are capable of uniformly growing to span an increased distance ofseveral micrometers (μm). Porous side-wall films 407 measure 700 nm inthickness. This thickness is well controllable by adjustment of the timelength of oxidation. After formation of films 407, the resist mask isthen removed away. Anode oxidation process is again carried out forminga thin, dense anode oxide film 408 covering island 409. This process maybe similar in condition to the above-described anode oxidation. Note inthis step that the anode oxide film 408 is formed by taking account ofthe fact that the electrolytic solution used attempts to enter or soakinto porous anode oxide films 407. Increasing the thickness of film 408up to 150 nm or greater may permit formation of a required off-set gateregion in a later step of injection of chosen impurity ions thereinto.Such dense film 408 will be able to function at a later step to suppressor eliminate occurrence of hillocks on the surface of a TFT gateelectrode (as will be denoted by numeral 409 later).

After formation of the thin dense anode oxide film 408, an impurity of achosen conductivity type—here, P type conductivity for manufacture of anN-channel TFT (NTFT)—is doped or injected by ion implantation into theunderlying patterned silicon film 403, thereby forming spaced-partheavily-doped regions 410, 411 as shown in FIG. 4C, which will act asthe source and drain of a TFT structure.

The porous anode oxide films 407 are selectively etched away usingchosen etchant of a mixture of acetic acid, phosphoric acid and nitricacid. Thereafter, P-type impurity ions are again injected into resultantstructure. The charge dose of this ion injection may typically be lessthan that for formation of the source and drain regions 410, 411 in film403. Spaced-part lightly-doped regions 412, 413 are thus defined in film403, which are in contact with the inner edges of heavily-doped sourceand drain regions 410, 411 as depicted in FIG. 4D, while allowing anintermediate region 414 between regions 412, 413 to be self-aligned withthe overlying gate electrode island 409. The intermediate region 414 isas a channel region for TFT structure.

After impurity injection the structure of FIG. 4D is then subject toannealing treatment by irradiation of a laser beam, infrared beam orultraviolet (UV) beam. In this way, the basic fundamental TFT structurecalled the “lightly-doped drain (LDD)” structure is obtained which comeswith the source region 410, lightly-doped regions (LDD regions) 412,413, channel region 414, and drain region 411.

It is recommendable at this stage that plasma hydrogenation treatment beperformed at temperatures of from 300 to 350° C. for 0.5 to 1 hour. Thisprocess is for doping hydrogen into the active layer 403 at apredetermined concentration, such as 5 at % (1×10²¹ atoms/cm³ or less);preferably, 1×10¹⁵ to 1×10²¹ atoms/cm³ or less. The doped hydrogen canneutralize and remove the unpaired coupling hands of silicon atoms inactive film 403 or the level of an interface between the active layerand gate insulating film.

Next, as shown in FIG. 4E, a dielectric film 415 is deposited as aninterlayer insulation layer on the structure of FIG. 4D. Film 415 may bemade of silicon oxide, silicon nitride, silicon oxide/nitride, resin orany multi-layered combination thereof. The use of silicon nitride ispreferable due to the capability of elimination of rediffusing out ofhydrogen doped at the previous step toward exterior of the devicestructure. Interlayer insulation film 415 is then patterned definingopenings that act as contact holes for electrical interconnectionsrequired. Metallic layers 416, 417 are next deposited to fill thesecontact holes with metal providing source and drain electrodes of TFT.

In cases where this TFT is for use as a pixel transistor inactive-matrix LCDs, there is not required any take-out or pad electrodefor applying a potential to the gate electrode 409; alternatively, wherethe TFT is for use in peripheral driver circuitry, it will be requiredthat a takeout or pad electrode electrically associated with gate 409 beformed simultaneously. The resultant structure is thereafter subject tohydrogenation by execution of heat treatment at 350° C. in the hydrogengas atmosphere. A TFT structure is thus completed as shown in FIG. 4E.

The TFT structure thus fabricated may offer field-effect carriermobility excellent enough to attain high speed switching operationsrequired. This is due to the fact that its active layer is fullycomprised of the mono-domain region. The reliability can also beenhanced since there are no substantive grain boundaries in the channelregion and at the drain contact while eliminating segregation of nickelcompounds thereinto.

EXAMPLE 3

An explanation will now be given of a significant advantage of thethermal oxidation process in the atmosphere containing halogen elementsfor formation of the mono-domain region as has been discussed previouslyin Example 1.

See FIG. 6. This diagram is a graph showing the relation of vaporpressure of nickel chloride (NiCl₂) versus temperature. As shown, sinceNiCl₂ is a sublimative material, Ni in the mono-domain crystallinesilicon film 111 of FIG. 2C may exhibit sublimation in nature as soon asit is gettered by chlorine. Resultant nickel chloride compounds will bereleased from the crystalline silicon film by outdiffusing into the airor by being absorbed by its overlying thermal oxide film. Thisadvantageously serves to enable successful removal of Ni from thesilicon film.

Electrical characteristics of the TFT structure of FIG. 4E will bedescribed with reference to FIG. 5, which demonstrates the relation ofgate current (Vg) versus drain voltage (Id) of the TFT. In this graphtwo Vg-Id characteristic curves are plotted: One curve 501 is that ofthe TFT of FIG. 4E in accordance with the invention; the other 502 is ofa standard TFT as fabricated without the thermal or heat treatment andnitride anneal steps.

Comparing the two transistor characteristics 501, 502 reveals the factthat a turn-on current flowing in the TFT of the present invention isgreater by two to four orders of magnitude than that of the standardTFT. The turn-on current refers to a drain current that attempts to flowwhile TFT is rendered conductive upon application of a gate voltage of 0to 5 volts as shown in FIG. 5.

It can also be seen from viewing the graph of FIG. 5 that the TFT ofFIG. 4E is greater in sub-threshold characteristic than the standardTFT. The “sub-threshold” characteristic, as used herein, is a measurerepresentative of the sharpness of TFT switching operations: As askilled person readily recognizes, the more sharp the rising angle ofId-Vg curve when TFT switches from the off to the on state, the betterthe subthreshold characteristics.

Attention should further be paid to the fact that while the subthresholdcharacteristics of the standard TFT remains around 350 mV/decade, thatof the present invention is as low as approximately 100 mV/decade. Thistells that the TFT of the invention is also enhanced in switchingperformance. In regard to the field-effect carrier mobility which servesas a parameter for estimation of TFT operation speed, the standard TFTis 80 to 100 cm²/Vs whereas the present invention is as large as 180 to200 cm²/Vs. This means that the latter can operate at high speedsaccordingly. From the foregoing, it can be experimentally demonstratedthat the TFT structure of the instant invention is capable of being muchimproved in electrical characteristics.

EXAMPLE 4

Our experimentation demonstrates a significant advantage of the getteraction of metallic element using chlorine as will be set forth below inconnection with the TFT structure of FIG. 4E.

See FIG. 7, which is a graphical representation of an experimentalresult indicative of the concentration distribution of the crystallinesilicon film along the profile thereof, as measured using the SIMSanalysis. Note here that measurement data with respect to certain regionclose in position to the film surface may be somewhat insignificant dueto presence of the risk of affection or influence from possible surfaceirregularity and absorbed residual objects therein. Note also that forthe same reason, data near interfaces will possibly involve errors. Ascan be seen from the graph of FIG. 7, chlorine is much present at ornear the interface between crystalline silicon film and its overlyingthermal oxide film. It is likely that this is because chlorine absorbedin the surface of crystalline silicon film at the beginning of the heattreatment outdiffuses into the thermal oxide film with Ni gettered. Itis also considered to suggest that a number of unpaired coupling handscalled the “dangling bonds” which have been at the surface of thecrystalline silicon film prior to formation- of the thermal oxide filmare terminated with those of chlorine.

EXAMPLE 5

One characterizing feature of the TFT-SOI structure having asingle-crystal on a silicon substrate with a silicon oxide film beinglaid therebetween is the capability of successfully suppressing oreliminating inclusion of any bad parameter elements which can affect ordisturb the crystal characteristics—such as pipe density, interfacelevel, fixed charge, penetration transition, and the like in resultantmono-domain crystalline active layers. More specifically, while the SOIstructure may exhibit an enhanced reduction in power dissipation as aresult of recent developments in the semiconductor technology, it stillsuffers from a problem. See FIG. 8. This diagram is an illustrationsummarizing several possible factors that can affect the crystallinityin one typical SOI structure: The level of interface and fixed charge ina silicon film, which are originated from the crystal structure; and,metal contamination and concentration of boron—these are due to externalaffection. Bad behavior of such factors can be minimized or eliminatedby the fabrication method of the present invention, which specificallyincludes the steps of heating the crystalline silicon film in theatmosphere containing halogen, thereby allowing bothsingle-crystallization of silicon film and gettering of metallic elementto be carried out at a time. Execution of the gettering process removesaway any possible metal contamination therein. This is mainly due to thehalogen elements' action, which may secondarily serve to increase innumber unpaired coupling hands of silicon atoms that have beendisengaged from nickel atoms. The single-crystallization by thermalannealing process may offer an advantage that any inferior factors canbe suppressed or eliminated such as irregularity of pipe density,interface level, fixed charge, penetration transition, and others.Insofar as the deposits or precipitates illustrated in FIG. 8 aresilicide-based materials, these can be removed away by the getter actionof halogen elements. If such are oxide materials, these will be expectedto disappear as a result of oxygen's reseparation for diffusing duringthe heat treatment.

EXAMPLE 6

One possible modification of the formation of the surface projection 103on the silicon oxide buffer layer 102 of FIG. 1A is shown in FIG. 9,wherein projection 103 is replaced with an elongate rectangular groovealthough this may still alternatively be an elongate projection of acorresponding planar shape. The resulting crystal grains grown areobtained in a manner similar to that of the embodiment shown in FIGS. 1Ato 1F.

As shown in FIG. 9, a lateral crystal growth region 902 is formed with avertical growth region 901 being as a crystal nucleus or seed. Adifference of region 902 from the region 302 of FIG. 3A is that the seedis in a linear shape rather than a pin-point shape. Due to suchdifference, the lateral growth region 902 exhibits elongated hexagon inplanar shape as seen from FIG. 9. As shown, this region 902 actuallygrown on a quartz substrate is subdividable into eight segments A to Hin such a manner that the opposite sets of segments A-C, F-H positionedalong the length Y of seed 901 are negligibly less in area than theremaining, two segments D and E which are at the opposite sides of seed901 along the width X thereof. This is due to the fact that the length Yof vertical growth seed 901 is sufficiently greater in length than itswidth X.

One advantage of this arrangement is that when such wide segments D, Eof FIG. 9 are single-crystallized, corresponding resultant mono-domainregions are increased in area. Employing these enlarged mono-domainregions for formation of TFT active layers may enable multiple activelayers to be defined in one mono-domain region having the same anduniform crystallinity.

EXAMPLE 7

A fabrication method of a complementary metal oxide semiconductor (CMOS)transistor is shown in FIGS. 12A to 14D, which make use of the TFTformation process of Example 2 although the present invention should notbe exclusively limited thereto.

As shown in FIG. 12A, a quartz substrate 31 is prepared on which asilicon oxide film 32 is deposited using the process earlier describedwith reference to FIGS. 1A-2C. Film 32 has a surface on which amono-domain crystalline silicon film is deposited using the technique ina manner similar to that as has been discussed previously. This siliconfilm is then patterned to define spaced-apart mono-domain active layers33, 34: One 33 is for the active layer of an N-channel TFT (NTFT); theother 34 is for that of a P-channel TFT (PTFT). Only two transistors aredepicted for purposes of explanation only; practically, when theinvention is reduced to practice, several millions of P- and N-channelTFTs are formed by microelectronics fabrication techniques on a singlechip substrate.

After formation of the mono-domain active layers 33, 34, an overlyinggate insulating film 35 is next deposited by plasma CVD techniques to apredetermined thickness, such as 50 to 200 nm, preferably 100 to 150 nm.Film 35 may be made of silicon oxide, silicon oxide/nitride, siliconnitride, or other available dielectric materials. The structure of FIG.12A is thus obtained.

Then, as shown in FIG. 12B, a conductive film 36 is deposited bysputtering or electron-beam deposition techniques over the structure ofFIG. 12A. Film 36 may be made of aluminum and will act as gateelectrodes of TFTs later. Film 36 contains therein scandium at 0.2weight percent (wt %) for elimination of occurrence of hillocks orwhisker. These refer to thorn-like or needle-like projections as createddue to abnormal crystal growth of aluminum. Presence of such projectionswill badly behave to cause unwanted short-circuit and crosstalk betweenadjacent wire leads or between laminated chip leads. Film 36 mayalternatively be made of anode-oxidizable metallic materials, includingtantalum.

At the step of FIG. 12B a thin dense film 37 is formed on the aluminumfilm 36 by anode oxidation process in electrolytic solution with film 36as the anode thereof. The electrolytic solution here isammonium-neutralized ethylene glycol solution containingdihydroxysuccinic acid at 3%. The use of such anode oxidation enablesformation of a uniform oxide film with enhanced density as well asthickness controllable by adjustment of a voltage as externally appliedthereto. Film 37 here measures 10 nm in thickness, and will play a roleof improving or enhancing the adhesion characteristics of a resist maskto be later formed thereon.

Next, as shown in FIG. 12C, a patterned photosensitive resist layerhaving mask segments 38, 39 is formed on the structure of FIG. 12B. Withphotoresist masks 38, 39, the underlying aluminum film 36 andanode-oxidation oxide film 37 are subject to patterning process toobtain a structure of FIG. 12C having correspondingly patterned filmsegments 40, 41. This structure is then subject to anode oxidationprocess with films 40, 41 being as the anode electrode During thisprocess the anode oxidation selectively progresses only at the sidewalls of each film 40, 41. This is because of the fact that a laminationof the dense film 37 and mask segments 38, 39 resides on the uppersurface of films 40, 41. As a result, porous oxide films 42, 43 aregrown to a thickness of several micrometers on the side walls of films40, 41. The progress distance of such anode oxidation—i.e., thethickness of side-wall oxide films 42, 43—is 700 nm, by way of example.The anode oxidation distance will determine the length of lightly-dopedregions to be formed later. Our experimentation suggests that thethickness of films 42, 43 preferably falls within a range of from 600 to800 nm. At this stage the structure of FIG. 12D is with gate electrodes1, 2 as shown.

After the resist masks 38, 39 are removed away, the structure of FIG.12D is again subjected to the anode oxidation using similar electrolyticsolution. During this process the solution attempts to enter and fillthe inside of porous side-wall oxide films 42, 43. Dense side-walloxides 44, 45 are thus formed as shown in FIG. 12E. These oxides 44, 45are typically 50 to 400 nm thick. This thickness is controllable byadjustment of externally applied voltages. Any residual portions of theearlier formed dense oxides 37 become integral with oxides 44, 45.

At the step of FIG. 12E the resultant structure is doped with an N typeimpurity such as phosphorus (P) over the entire surface thereof. Thecharge dose is as high as 2×10¹⁴ to 5×10¹⁵ cm⁻²; preferably, the dosemay range from 1 to 2×10¹⁵ cm⁻². Known plasma- or ion-implantationtechniques may be employed. As a result, heavily-doped regions 46-49 aredefined in the mono-domain active layers 33, 34 as shown in FIG. 12E.One pair 46, 47 is self-aligned with its corresponding gate electrode 1having side-wall oxides 42; the other pair 48-49 is self-aligned withgate electrode 2 having side-wall oxides 43.

Thereafter, the side-wall oxide films 42, 43 are removed using chosenetchant of aluminum-mixed acid. At this time the active regions justbeneath oxides 42, 43 remain essentially intrinsically pure in naturedue to inhibition of any ion doping thereinto.

After removal of oxides 42, 43 a photoresist mask layer 50 isselectively formed covering the right-hand surface area whereat a PTFTwill be formed as shown in FIG. 13A. The left-hand surface area of thestructure of FIG. 13A is kept exposed as shown.

Then, as shown in FIG. 13B, the structure is doped with a P-typeimpurity at a relatively low charge dose as compared to that at the stepof FIG. 12E, such as 1×10¹³ to 5×10¹⁴ cm⁻²; preferably, 3×10¹³ to 1×10¹⁴cm⁻². As a result of such impurity doping, spaced-apart lightly-dopedregions 52, 54 are defined at selected portions of the mono-domainactive layer 33, which have been located beneath the side-wall oxides 42now removed away. These regions 52, 54 are self-aligned with the gateelectrode 1 as shown. Heavily-doped regions 51, 55 are also defined atouter locations of active layer 33 in such a manner that region 51 is incontact with region 52 whereas region 55 is with region 54. These outerheavily-doped regions 51, 55 will act as the source and drain of NTFT,respectively. Inner lightly-doped regions 52, 54 laterally lie at theopposite ends of an intrinsic channel formation region 53, which isself-aligned with the gate electrode 1. One region 54 which ispositioned between channel region 53 and drain 55 acts as the so-called“lightly-doped drain (LDD)” region.

It should be noted in FIG. 13B that non-doped regions (not shown) existbetween the channel 53 and lightly-doped regions 52, 54 as originatedfrom the fact that the presence of thin oxide 44 covering the surface ofgate electrode 1 eliminates ion injection thereinto during the impuritydoping. Such non-doped regions are equivalent in width to the thicknessof oxide 44, and are generally called the “off-set gate” regions in theart to which the invention pertains. The offset gate regions areessentially intrinsic with no impurity doped thereinto; however, in theabsence of gate voltages, they do not contribute to formation of achannel and therefore function as a resistance component which weakensthe intensity of internal electric field to suppress or eliminatedeterioration of material quality increasing the net life of TFTs. Notehere that where the offset width is decreased, resultant offset regionswill no longer exhibit such functions. Any quantitative analysis thereonhas not been completely established yet.

After formation of the NTFT, as shown in FIG. 13C, the resist 50 isremoved, and another patterned photoresist layer 56 is then depositedcovering the NTFT at the left-hand side in the illustration. With thisresist 56 being as a mask, a P-type impurity, such as boron (B), isdoped into the structure of FIG. 13C. The charge dose is 2×10¹⁴ to1×10¹⁶ cm⁻²; preferably, 1 to 2×10¹⁵ cm⁻², although it may alternativelybe the same as that at the step of FIG. 12E if appropriate. Dopedregions 57, 61 are thus defined at opposite sides of the mono-domainactive layer 34. While these regions may contain both N-type and P-typeimpurities, these essentially function as contact pads for electricalinterconnection with associated pad electrodes to be coupled to chipleads. In other words, unlike the left-hand side NTFT structure, thePTFT functionally distinguishes the regions 57, 61 from its source anddrain regions. In this respect, it will be seen that the source anddrain of PTFT consist of other doped regions 58, 60 as self-aligned withits corresponding gate electrode 2, respectively. These regions 58, 60have been defined by injecting only B ions into the locations that havebeen essentially intrinsic in nature. For this very reason any otherions do not exist here facilitating the controllability of impurityconcentration, which in turn enables achievement of PI junctionsexcellent in crystal-lattice matching property while reducing crystalirregularity otherwise occurring due to ion injection. Note that theformation of offset gate regions remains available by use of the oxidefilm 45 covering the surface of gate 2 if required in some cases;however, the illustrative structure does not come with such offsetregions by taking account of the fact that our experimentation revealedthat PTFTs will hardly degrade as compared to NTFTs.

In this way, as shown in FIG. 13C, the source and drain regions 58, 60are formed in the mono-domain active layer 34 of PTFT. An intermediatenon-doped region laid between source and drain 58, 60 defines a channelformation region. The doped-regions 57, 61 at the opposite side portionsof active layer 34 will act as contact pads for supplying current tosource 58 or deriving it from drain 60.

After the resist 56 is removed, as shown in FIG. 13D, resultantstructure is then exposed to a laser beam for activation of dopedimpurity as well as annealing of doped-regions. The laser irradiationmay be carried out while reducing a difference in crystallinity betweena pair of source and drain regions 51, 55 of NTFT and another pair ofsource and drain regions 58, 60 of PTFT. Absence of clear difference ofcrystallinity therebetween is originated from the fact that source anddrain regions 58, 60 are not significantly damaged during the ioninjection at the step of FIG. 13C. Accordingly, the laser annealing maycure the doped source and drain regions of the both TFTs to ensure thatP- and N-channel TFTs are similar or identical in transistorcharacteristics to each other.

Next, as shown in FIG. 14A, an interlayer dielectric film 62 isdeposited, by plasma or thermal CVD techniques, to a thickness of 400 nmon the entire surface of the structure of FIG. 13D. Film 62 may be madeof silicon oxide, silicon oxide/nitride, silicon nitride, or anycombinations thereof in a multilayer manner.

Finally, as shown in FIG. 14B, several required openings are defined ascontact holes in the interlayer film 62. Patterned conductive films63-66 are then selectively formed to fill the contact holes to act assource and drain electrodes of P- and N-channel TFTs. A chip leadpattern is also formed allowing the drain electrode 64 of NTFT to beelectrically coupled to that 66 of PTFT while permitting interconnectionbetween the insulated gate electrodes 1, 2 thereof. A CMOSTFT structureis thus obtained, which is applicable to advancedhigh-speed/high-precision display panels, including active-matrix LCDs,active-matrix electro-luminescence (EL) devices, and others.

One significant importance of the illustrative TFT manufacture scheme isthat at the steps of FIGS. 12E, 13B and 13C, the mono-domain activelayers 33, 34 are completely covered on surface by the silicon oxidefilm 35 which later acts as the gate insulating films after patterning.Performing ion-doping with active layers 33, 34 covered by oxide 35 mayadvantageously serve to reduce the risk of occurrence of irregularityand residual contamination on the active layer surface. This willgreatly contribute to an increase in production yield as well asreliability of resultant TFTs.

EXAMPLE 8

It should be noted that the mono-domain crystalline silicon film 111 asshown in FIG. 2C may alteratively be fabricated on a semiconductivesubstrate such as a silicon wafer. In this case it is required that anadditional dielectric film be deposited on the top surface of thesubstrate. A currently available thermal oxide film may be employed forthis purpose. A heat treatment therefor is carried out at temperaturestypically ranging from 700 to 1300° C. for a predefined length of timeperiod, which may vary with a change in target thickness. The thermaloxidization process is done in chosen atmosphere that burns O₂, O₂—H₂O,H₂O, O₂—H₂. Recent advance in the semiconductor art suggests that theoxidization may alternatively be done in the atmosphere containingtherein chosen halogen elements, such as HCl, Cl₂ or the like. Sincesilicon wafers are a key to the recent semiconductor microfabricationtechnology due to the extended capability of forming thereon severaltypes of semiconductor elements. Forming the mono-domain silicon film onsuch semiconductor wafers may further extend the applicability of thepresent invention in combination of the presently availablesilicon-wafer fabrication techniques.

Turning now to FIG. 15A to 15B, a fabrication method of the mono-domaincrystalline silicon film in accordance with a further embodiment of theinvention is shown which is designed to form a TFT structure with suchfilm being laid over an integrated circuit (IC) preformed on a siliconwafer under manufacture.

In FIG. 15A, there is shown (not drawn to scale) a MOSFET IC devicewhich has been fabricated using known microfabrication techniques. ThisIC comes with a silicon substrate 71 having a top surface on whichMOSFETs are formed along with associated element-separation dielectriclayers 72, 73 as typically formed in a thermal oxide film. A MOSFET hasspaced-apart source and drain regions 74, 75 in the surface of substrate71. These may be fabricated through the injection step of doping animpurity of a selected conductivity type into substrate 71 and itsfollowing diffusion step. As well known, where substrate 71 is of Pconductivity type, an N-type impurity such as phosphorus (P) is chosenfor injection; if substrate 71 is of N type then a P-type impurity suchas boron (B) is doped thereinto. The MOSFET also has a channel formationregion 76 as defined between the source 74 and drain 75 in the substratesurface, and an insulated gate electrode 77 overlying the channel 76.Gate electrode 77 may be made of polycrystalline silicon.

Gate 77 is electrically insulated from substrate 71 by a gate insulatingfilm which is sandwiched therebetween. This film may be a residualportion of a thermal oxide film as has been formed with thicknesscontrol during the diffusion step after ion injection for forming source74 and drain 75. Gate 77 is covered by a silicon oxide film 78 forelectrical isolation from a source electrode 79, a drain electrode 80 orother adjacent components on substrate 71.

As shown in FIG. 15B, an interlayer dielectric film 81 is deposited onthe MOSFET-IC structure of FIG. 15A. Film 81 may be made of siliconoxide, silicon nitride; or others. A contact hole is defined in film 81at a selected location. A patterned conductive wiring layer 82 is thenformed as a chip lead, permitting electrical interconnection of drainelectrode 80 to any required part or parts of the IC.

The structure of FIG. 13B is next subjected to surface polishing processusing known chemical/mechanical polishing (CMP) techniques, obtaining asurface-flattened IC structure shown in FIG. 15C. As shown, due to suchsurface polish treatment, a resultant interlayer dielectric layer 83exhibits flat, smooth top surface 84 with any undesired projections oflead 82 being removed away from it. In FIG. 13C numeral 85 is used todesignate flattened portion of lead 82, on which a chip lead 86 isformed for interconnection with drain electrode 80. It is recommendablethat the source electrode 79, drain electrode 80 and lead 86 be made ofa carefully chosen heat-resistant material which is capable ofwithstanding against application of heat as increased up to 1100° C.This is in view of later heat application(s) during formation of amono-domain crystalline active layer.

Next, as shown in FIG. 15D, an interlayer dielectric film 87 isdeposited on the entire surface of the structure of FIG. 15C. Amono-domain crystalline silicon film which acts as the active layer of aTFT will be formed on this film 87. The formation of such active layeris similar in principle to that shown in FIGS. 1A-2C. More specifically,a patterned mono-domain crystalline silicon active layer 88 is formed onfilm 87. A gate insulating film 89 is deposited covering film 87 andactive layer 88. A gate electrode 90 is then formed insulatinglyoverlying a channel region of active layer 88. A chosen impurity of aselected conductivity type is next doped into active layer 88.

After injection of impurity, insulators 91 are selectively formed on theopposite side walls of gate electrode 90. Formation of such side-wallinsulators 91 includes the step of depositing a silicon oxide film (notshown) which is greater in thickness than gate 90 and which covers theentire surface thereof, and performing anisotropic dry etching to removeselected portions of such insulating film, thereby causing insulators tobe reside only at the opposite side walls of gate 90 as shown.

A further injection of impurity is performed defining in active layer 88the heavily-doped source and drain regions while allowing certain partsshielded by side-wall insulators 91 to remain as lightly-doped regions.Impurity activation process is then carried out using heat treatmentand/or laser-beam irradiation. Thereafter, a dielectric film which maybe made of silicon oxide or silicon nitride is deposited as theinterlayer insulation layer. This layer is subject to etching process,forming contact holes therein. Finally, source and drain electrodes 93,94 are formed providing electrical interconnections of source and drainin active layer 88 through the contact holes.

A significant advantage of the embodiment shown in FIGS. 15A to 15D isthat a multiple-layered or “three-dimensional (3D)” structured TFT canbe fabricated on or above the presently available IC devices.Specifically, with the 3D MOS-IC/TFT structure of FIG. 15D, the upperTFT can exhibit extra enhanced transistor actions that may be equivalentin speed and reliability to the lower standard MOSICs as fabricated onsingle-crystalline base plate such as silicon wafer or substrate 71depicted. This advantageously serves to offer an increased integrationor packing density for IC devices without having to reduce theirinherent performance.

EXAMPLE 10

A dynamic random access memory (DRAM) device embodying the presentinvention is shown in FIGS. 14A and 14B, which employs the TFT structureof the invention. The DRAM includes an array ofone-capacitor/one-transistor memory cells, one of which is shown in FIG.16A. As shown, the memory cell includes a data transfer transistor 1603having a gate coupled to a corresponding one of parallel word lines1601, a source coupled to a corresponding bit line 1602, and a drain.Transistor 1603 is a TFT with an active layer as made of the mono-domaincrystalline silicon film as has been described previously. The cell alsoincludes an associative data storage capacitor 1604 having one electrodecoupled to the drain of TFT 1603, and the remaining electrode as coupledto a fixed potential, such as ground. In the DRAM cell of FIG. 16A, uponapplication of a voltage signal of a selected potential at the word line1601, this potential is applied to the gate rendering TFT 1603conductive. This allows a data signal to be transferred from bit line1602 through TFT 1603 to capacitor 1604 causing corresponding chargecarriers to be accumulated or integrated therein for data write. Duringread operation the stored carriers are transferred via TFT 1603 to bitline 1602.

See FIG. 16B, which shows a cross-sectional view of the DRAM cell ofFIG. 16A. As shown, a quartz or silicon substrate 1605 has a top surfaceon which a silicon oxide film 1606 is formed. In the case of using asilicon substrate, the so-called semiconductor-on-insulator (SOI)structure can be used. Film 1606 may be a thermal oxide layer. Formed onfilm 1606 is a TFT having a mono-domain crystalline silicon active layer1607 in accordance with the principle of the present invention.

As apparent from FIG. 16B, the active layer 1607 is covered with anoverlying gate insulating film 1608, on which an insulated gateelectrode 1609 is arranged. An interlayer insulation film 1610 islaminated on film 1608 covering gate 1609. Film 1610 has a contact holethrough which a source electrode 1611 is electrically coupled to thesource region in active layer 1607 in a manner similar to those of theprevious embodiments. Source electrode 1611 is also coupled to acorresponding bit line 1602 of FIG. 16B. Another conductive layer 1612is also on interlayer insulation film 1610 as one electrode of the datastorage capacitor 1604 of FIG. 16B, which defines a predefinedcapacitance between it and the underlying drain region of TFT in activelayer 1607. Source electrode 1611, capacitor electrode 1610 and bit line1602 are formed at a time. An insulating layer 1613 covers the entiretop surface of the cell as a protective layer.

A significant feature of the embodiment shown FIGS. 16A and 16B is thatleak current can be suppressed. This can be said because the TFT 1603 isemployed to form the SOI structure in the low-cost/high-integrationone-capacitor/one-transistor DRAM cell minimizing the junction area,which in turn leads to an increase in data-storage reliability.

Another advantage is to enable achievement of low-voltage operations dueto the fact that the SOI-DRAM cell structure permits a decrease in thestorage capacitance without reducing reliability and performance.

EXAMPLE 11

A static random access memory (SRAM) device also embodying the presentinvention is shown in FIGS. 17A and 17B, which employs the TFT structureof the invention. The SRAM includes an array of NMOS or CMOS memorycells each of which has bistable flip-flop (F/F) circuitry as shown inFIG. 16. The SRAM cell statically stores therein a binary one-bit dataof logic “0” or “1” depending upon whether the F/F circuit turns on oroff insofar as application of power continues. As shown in FIG. 17A, thecell is at an intersection between a word line 1701 and a pair of bitlines 1702, and includes a F/F circuit which is constituted from a pairof cross-coupled driver transistors 1704, and associativehigh-resistance load elements 1703. One pair of load 1703 and transistor1704 is interconnected at a common node to one bit line 1702 via anaccess transistor 1705 having a gate coupled to the word line 1701; theother pair of load and transistor is connected to the other bit line1702 through a similar access transistor 1705.

A cross-sectional view of a TFT for use in the SRAM cell is shown inFIG. 17B. A substrate 1706 may be made of quartz or silicon. A siliconoxide film 1707 is on substrate 1706 as the primary coat layer on whicha mono-domain crystalline silicon active layer 1708 of the TFT isformed. Active layer 1708 is covered by a gate insulating film 1709, onwhich a patterned gate electrode 1710 is formed. An overlying interlayerdielectric film 1711 has contact holes through which source and drainelectrode 1712, 1713 are electrically coupled to the source and drainregions in active layer 1708 as defined in the manner as describedpreviously. Source and drain electrodes 1712, 1713 are fabricated alongwith bit lines 1702. An interlayer dielectric film 1714 and apolycrystalline film 1715 are laminated in this order. The latter film1715 acts as the high-resistance load element 1703 of FIG. 17A. Theoverall multilayer structure is covered by a protective film 1716 madeof a chosen dielectric material. With such an arrangement, the SRAM cellcan exhibit high speed operation with reliability and mountabilitymaximized due to the use of the TFT with mono-domain active layer 1708as fabricated on the SOI substrate structure.

EXAMPLE 12

A further embodiment of the invention is drawn to a combination of thesemiconductor device shown in FIG. 4E (EXAMPLE 2) and CMOS structure ofFIG. 14B (EXAMPLE 7) to provide an active-matrix LCD device which has anarray of rows and columns of active-matrix TFT pixels and an associateddriver circuitry as integrated on a single chip substrate. Morespecifically, the pixel array makes use of at least one TFT for theindividual one of these pixels. The driver circuitry is disposed at theperiphery of the substrate surface to surround the TFT pixel array. TheTFT structure of FIG. 4E which is equivalent in performance tosingle-crystalline MOSFETs is employed as such pixel TFTs whereas theCMOSTFT of FIG. 14B is as the driver TFTs.

A significant advantage of this active-matrix LCD device is that theturn-off current in pixel transistor can be reduced or minimized. Thereason of this is as follows: Since the TFT active layer consists of themono-domain crystalline silicon film as mentioned previously, there areno longer present any crystal grain boundaries otherwise serving tocreate a current path through which the turn-off current can rash toflow at increased priority. This in turn increases retainability of apacket of signal charge at the individual pixel electrode.

Another advantage of this embodiment is that the CMOSTFT drivercircuitry can be enhanced in performance as well as in equalization oftransistor characteristics between PMOSTFTs and NMOSTFTs, by use of theCMOSTFT structure of FIG. 14B.

EXAMPLE 13

The fabrication method shown in FIGS. 4A-4E may be modified in formationof the gate insulating film as follows. After manufacture of themono-domain crystalline silicon film 111 of FIG. 2C, a TFT active layeris then formed by selectively using the mono-domain region only. Adielectric thin film containing silicon as its major component—here,silicon oxide—is deposited on the active layer by vapor-phase methodsuch as CVD or PVD techniques to a predetermined thickness of 20 to 150nm, preferably 80 nm. The thickness of such silicon oxide may besuitably designed in view of the dielectric withstanding characteristicsas finally required. The silicon oxide may be replaced with otherequivalent materials including silicon oxide/nitride, silicon nitride,and others.

After completion of the silicon oxide film, the resultant structure isagain subject to heating in the atmosphere containing halogen elements.This heating is called the “third heat treatment” hereinafter. The thirdheat treatment is similar in condition to the prior second heattreatment as executed in the embodiment of FIGS. 1A-2C.

During this third heat treatment, the metallic element such as nickelwhich remains within the active layer is further reduced in contentimproving the crystallinity of the mono-domain region accordingly.During this process the thermal oxidation reaction progresses at theinterface between the active layer and silicon oxide film forming athermal oxide film of 20-nm thick. In this case it will be recommendablethat the final thickness of active layer fall within a range of from 20to 30 nm, preferably at 25 nm. This may advantageously serve to reduceor minimize the turn-off current in magnitude.

After completion of the third heat treatment, the resultant structure issubjected to a still further heat treatment at 950° C. for one hour inthe atmosphere of nitride gas, for curing any possible heat damage ofthe thermal oxide and silicon oxide films to improve the film quality.Furthermore, as a result of the heat treatment in the atmospherecontaining halogen, the halogen can reside at an increased concentrationnear the interface between the active layer and an overlying gateinsulating film. It is shown by our SIMS experimentation that theconcentration of halogen ranges from 1×10¹⁹ to 1×10²⁰ atoms/cm³. Thethermal oxide film as formed at the interface between the active layerand silicon oxide will be used to constitute the gate insulating filmalong with the silicon oxide film. Any defective levels and interlatticesilicon atoms are reduced during formation of the thermal oxide film,enhancing the resulting interface state between the active layer and thegate insulating film. As has been described in connection with theembodiment of FIGS. 1A-2C, the active layer offers a maximized flatnesson its top surface; accordingly, the thermal oxidation reactionprogresses regularly rendering the gate insulating film uniform inthickness. This improves the interface state while enhancing thewithstanding or breakdown voltage characteristics of the gate insulatingfilm.

An advantage of this embodiment is that the content of the metallicelement such as Ni can be reduced in the active layer while renderingthe interface excellent in state between the active layer and itsoverlying gate insulating film. This leads to the capability ofproviding semiconductor devices with electrical characteristics andreliability enhanced. Optionally, the second heat treatment of theembodiment shown in FIGS. 1A-2C and the third heat treatment of thisembodiment may be done simultaneously. To do this, the crystallinesilicon film 109 of FIG. 1F, which is prior to execution of the firstheat treatment, is patterned forming the active layer which is thensubject to the prescribed process of this embodiment.

EXAMPLE 14

One possible modification of the active-matrix LCD device using acombination of the semiconductor device shown in FIG. 4E and CMOSstructure of FIG. 14B is as follows. This example is a modification ofExample 2, aimed at an improvement in the interface state between theactive layer and gate insulating film.

After manufacture of the mono-domain crystalline silicon film 111 ofFIG. 2C, a TFT active layer is then formed by selectively using themono-domain region only. A silicon oxide film is deposited on the activelayer by vapor-phase method such as CVD or PVD techniques to apredetermined thickness of 20 to 150 nm.

After completion of the silicon oxide film, resultant structure isheated at temperatures of 500 to 700° C. (preferably, 640 to 650° C.).The temperature range is determined to be near the lower limittemperature for intended thermal oxidation. The heat treatment may bedone in certain atmosphere exclusively containing oxygen or containinghalogen. Alternatively, wet atmosphere containing moisture vapor may beused. The heat treatment is carried out for 0.5 to 2 hours forming athermal oxide film to a target thickness, for example, severalnanometers; typically, 1 to 9 nm. The growth of such thermal oxide willbe completed when its thickness becomes equivalent thereto.

An advantage of this approach is that good inference state can beobtained between the active layer and gate insulating film, by reducingor removing residual fixed charge or defective levels at or near thepolar interface. The reduction or absence of such defects is attained bythermally oxidizing only a limited shallow region of the top surfacesection of active layer, which region is 1 to 3 nm in depth orthickness. In other words, with this embodiment, excellent interfacestate can be accomplished by specifically forming a very thin thermaloxide film being limited in thickness. The oxidation here may refer torendering the active layer thinner by 1 to 3 nm while forming thereon anew thermal oxide film of 2 to 6 nm thick. One possible explanation forthe capability of obtaining such good interface is that the presence ofundesirable residual fixed charge and/or crystal defects tend toconcentrate exclusively in the above-identified shallow surface regionof the active layer which falls within a narrow region of 1 to 3 nmspanning the active layer and gate insulating film with the interfacebeing as a center; therefore, by removing and replacing the shallowsurface region with the thermal oxide, it becomes possible to avoidinclusion of such defects almost completely.

Another advantage of this approach is that the manufacture ofsemiconductor devices can be improved in efficiency—namely,throughput—due to the fact that the thermal oxidation process usedherein can be performed at relatively lower temperatures, reducing dutyof equipment as employed therefor.

EXAMPLE 15

Turning now to FIGS. 19A to 19D, there is shown a TFT fabricationprocess in accordance with a further embodiment of the invention, whichemploys polycrystalline silicon (polysilicon) film as the gate electrodeof a TFT under manufacture.

In FIG. 19A, an insulating substrate 1901 is prepared which may be madeof glass. The glass substrate 1901 has a top surface on which there aresequentially formed a primary coat film 1902, a patterned mono-domaincrystalline active layer 1903, a gate insulating film 1904, and apatterned gate electrode 1905. Active layer 1903 is fabricated using theembodiment process as previously discussed in connection with FIGS.1A-2C. Gate 1905 is made of polysilicon.

The structure of FIG. 19A is then doped with an impurity by ionimplantation techniques so that spaced-apart doped regions 1906, 1907are defined in active layer 1903 in such a manner that these areself-aligned with the overlying gate 1905 as shown in FIG. 19B. Then, asilicon nitride film 1908 is deposited to a thickness of 0.5 to 1 μm, bylow pressure CVD, plasma CVD or sputter techniques, on the resultantstructure. Film 1908 may alternatively be made of silicon oxide.

Then, the structure of FIG. 19B is subject to etch-back process toselectively etch the overlying film 1908 causing only parts of it toreside on the opposite side walls of gate 1905 as shown in FIG. 19C.These side-wall insulators are designated by numeral 1909 herein. Duringthe etching, the gate insulating film 1904 is also etched away, and mostof it other than certain part underlying a mask consisting of gateelectrode 1905 and side-wall insulators 1909 is removed away.

Next, the structure of FIG. 19C is again doped with a chosen impurity byion implantation. The charge dose here is greater than that at the priorstep of impurity ion implantation. During the second ion implantationcertain regions 1910, 1911 just beneath side-wall insulators 1909 arekept unchanged in impurity concentration due to the fact that noimpurity is implanted thereinto. The remaining, exposed regions 1912,1913 of active layer 1903 are further doped with impurity ions toincrease the concentration of doped impurity therein. Through the firstand second ion implantation steps, active layer 1903 comes to haveheavily-doped source and drain regions 1912, 1913 as well aslightly-doped LDD regions 1910, 1911 laid just beneath side-wallinsulators 1909. Active layer 1903 also has a non-doped intermediateregion 1914, which is just beneath gate 1905 and will act as a channelformation region in resultant TFT.

A titanium film (not shown) of 30 nm thick is formed on the structure ofFIG. 19C, causing it to chemically react with the silicon film. Afterthe titanium film is removed, resultant structure is heated by lampanneal techniques to form titanium-silicide films 1915-1917 on theexposed surface areas of source 1912, drain 1913 and gate 1905 as shownin FIG. 19D. The titanium film may be replaced with any one of tantalum,tungsten and molybdenum films. Then, a silicon oxide film 1918 isdeposited as the interlayer insulator to a thickness of 500 nm; next,several types of suitably patterned chip leads 1919-1921 for electricalinterconnection of source 1912, drain 1913 and gate 1905 are formed thuscompleting a TFT structure shown in FIG. 19D.

An advantage of this approach is that good ohmic contacts can beattained in the TFT structure because of the fact that electricalinterconnection between the TFT and chip leads are made viatitanium-silicide films 1915-1917.

EXAMPLE 16

Any one of the foregoing TFTs embodying the invention may be applicableto a wide variety of types of semiconductor devices, includingelectrooptical display panels such as active-matrix LCD, EL or ECdevices; memory devices such as DRAMs, SRAMs, VRAMs, SDRAMs, ROMs,PROMs, EEPROMs, Flash EEPROMs, NAND/NOR EEPROMs or the like; and anyother equivalents which will be employed for advanced electronicapparatus or systems, such as TV cameras, head-mount display modules,motor vehicle navigation systems, front- or rear-projection displayunits, home-use video cameras, personal computers and others.

See FIG. 20A; which depicts a mobile computer. This computer isgenerally structured from a main body 2001, a tuner section 2002, adisplay unit 2003, a control switch 2004, and a display unit 2005. TheTFT of the present invention may be applied to ICs being assembled indisplay unit 2005 and main body.

A head-mount display is shown in FIG. 20B. This display is generallystructured from a main body 2101, a display unit 2102, and a bandsection 2103. The display unit 2102 includes a pair of relativelysmall-size display panels.

A motor vehicle navigation apparatus is shown in FIG. 20C. As shown,this apparatus includes a main body 2201, a display unit 2202, controlswitches 2203, and antenna 2204. The semiconductor device of the instantinvention may be applied as ICs for use in display unit 2201 andinternal as built-in electronics. The display unit 2202 acts as amonitor for purposes of visual indication of road map images thereon;accordingly, this may be relatively extensive in allowable range ofresolution.

A portable or handheld mobile telephony is shown in FIG. 20D, whichcomes with a main body 2301, an audio output section 2302, an audioinput section 2303, a display unit 2304, control switches 2305, and anantenna 2306. The semiconductor device of the instant invention may beapplied as ICs for use in display unit 2301 and built-in electronics.

A video camera is shown in FIG. 20E, which includes a main body 2401, adisplay unit 2402, an audio input section 2403, control switches 2404, abattery pack 2405, and a picture receiver 2406. The semiconductor deviceof the invention may be applied as ICs for use in display unit 2402 andbuilt-in electronics.

A front projection apparatus is shown in FIG. 20F, which may beconstituted from a main body 2501, a light source 2502, a reflectiontype display unit 2503, an optical system 2504 (including knownbeam-splitters, optical polarizers and the like), and an associatedscreen 2505. The screen 2505 is a large-size one adaptable for use inpresentations for the meetings and academic conferences; it is thusrequired that the display unit 2503 be high in resolution.

The semiconductor device of the invention will also be applicable to anytypes of electrooptical modules or apparatus other than the illustrativeones, including rear-projection systems, portable electronic intelligenttools such as handy terminals. As is apparent from the foregoing, thepresent invention may offer increased applicability, covering almost allof the currently available electronic display systems.

It has been described that the present invention may enable formation orfabrication of enlarged mono-domain regions with enhanced grainsize-controllability by intentionally forming a cite acting as a crystalnucleus or seed for crystal growth while performing the heat treatmentsin the atmosphere containing halogen. With such an arrangement, it ispossible to form an intended mono-domain region or regions on asubstrate having a dielectric surface, which regions can besubstantially identical in crystal structure to single-crystallinematerials. This in turn enables achievement of a superior active layerof semiconductor devices such as TFTs by use of crystalline siliconfilms having the crystallinity equivalent to single-crystals. This makesit possible to constitute semiconductor circuitry with enhancedperformance equivalent to that of ICs as manufactured using knownsingle-crystalline wafers.

While the invention has been particularly shown and described withreference to preferred embodiments thereof, it will be understood bythose skilled in the art that the foregoing and other changes in formand details may be made therein without departing from the spirit andscope of the invention.

1. An SRAM comprising: a substrate; an insulating film formed on thesubstrate, said insulating film having a protrusion; a pair ofcross-coupled driver transistors formed over the substrate; a pair ofaccess transistors; a pair of bit lines electrically connected to thecross-coupled driver transistors through the access transistors,respectively; and a word line electrically connected to the pair ofaccess transistors, wherein at least one of the access transistorscomprises a crystalline semiconductor film formed on the insulatingfilm, said crystalline semiconductor film having a mono-domain region inwhich a channel formation region is formed, and wherein said crystallinesemiconductor film comprises a source region and a drain region, and ametallic silicide is formed on the surface of said source region andsaid drain region.
 2. The SRAM according to claim 1 wherein saidmono-domain region includes substantially no grain boundary.
 3. The SRAMaccording to claim 1 wherein any grain boundary included in saidmono-domain region is electrically inactive.
 4. An electronic apparatuscomprising the SRAM according to claim 1, wherein the electronicapparatus is selected from the group consisting of a head-mount display,a motor vehicle navigation, a mobile phone, a video camera, a projector,and a mobile computer, wherein the electronic apparatus.
 5. An SRAMcomprising: a substrate; an insulating film formed on the substrate,said insulating film having a protrusion extending in one direction; apair of cross-coupled driver transistors formed over the substrate; apair of access transistors; a pair of bit lines electrically connectedto the cross-coupled driver transistors through the access transistors,respectively; and a word line electrically connected to the pair ofaccess transistors, wherein at least one of the cross-coupled drivertransistors comprises a crystalline semiconductor film formed on theinsulating film, said crystalline semiconductor film having amono-domain region in which a channel formation region is formed, andwherein said crystalline semiconductor film comprises a source regionand a drain region, and a metallic silicide is formed on the surface ofsaid source region and said drain region.
 6. The SRAM according to claim5 wherein said mono-domain region includes substantially no grainboundary.
 7. The SRAM according to claim 5 wherein any grain boundaryincluded in said mono-domain region is electrically inactive.
 8. Anelectronic apparatus comprising the SRAM according to claim 5, whereinthe electronic apparatus is selected from the group consisting of ahead-mount display, a motor vehicle navigation, a mobile phone, a videocamera, a projector, and a mobile computer, wherein the electronicapparatus.
 9. An SRAM comprising: a substrate; an insulating film formedon the substrate, said insulating film having a protrusion extending inone direction; a pair of cross-coupled driver transistors formed overthe substrate; a pair of access transistors; a pair of bit lineselectrically connected to the cross-coupled driver transistors throughthe access transistors, respectively; and a word line electricallyconnected to the pair of access transistors, wherein at least one of theaccess transistors comprises a crystalline semiconductor film formed onthe insulating film, said crystalline semiconductor film having amono-domain region in which a channel formation region is formed, andwherein said crystalline semiconductor film comprises a source regionand a drain region, and a metallic silicide is formed on the surface ofsaid source region and said drain region.
 10. The SRAM according toclaim 9 wherein said mono-domain region includes substantially no grainboundary.
 11. The SRAM according to claim 9 wherein any grain boundaryincluded in said mono-domain region is electrically inactive.
 12. Anelectronic apparatus comprising the SRAM according to claim 9, whereinthe electronic apparatus is selected from the group consisting of ahead-mount display, a motor vehicle navigation, a mobile phone, a videocamera, a projector, and a mobile computer, wherein the electronicapparatus.
 13. An SRAM comprising: a first layer comprising at least onefirst transistor and an insulating film formed over the firsttransistor; a second layer on the first insulating film; wherein thesecond layer comprises: a pair of cross-coupled driver transistors; apair of access transistors; a pair of bit lines electrically connectedto the cross-coupled driver transistors through the access transistors,respectively; and a word line electrically connected to the pair ofaccess transistors, wherein at least one of the access transistorscomprises a crystalline semiconductor film formed on the insulatingfilm, said crystalline semiconductor film having a mono-domain region inwhich a channel formation region is formed, and wherein said crystallinesemiconductor film comprises a source region and a drain region.
 14. TheSRAM according to claim 13 wherein said mono-domain region includessubstantially no grain boundary.
 15. The SRAM according to claim 13wherein any grain boundary included in said mono-domain region iselectrically inactive.
 16. An electronic apparatus comprising the SRAMaccording to claim 13, wherein the electronic apparatus is selected fromthe group consisting of a head-mount display, a motor vehiclenavigation, a mobile phone, a video camera, a projector, and a mobilecomputer, wherein the electronic apparatus.
 17. An SRAM comprising: afirst layer comprising at least one first transistor and an insulatingfilm formed over the first transistor; a second layer on the firstinsulating film; wherein the second layer comprises: a pair ofcross-coupled driver transistors; a pair of access transistors; a pairof bit lines electrically connected to the cross-coupled drivertransistors through the access transistors, respectively; and a wordline electrically connected to the pair of access transistors, whereinat least one of the cross-coupled driver transistors comprises acrystalline semiconductor film formed on the insulating film, saidcrystalline semiconductor film having a mono-domain region in which achannel formation region is formed, and wherein said crystallinesemiconductor film comprises a source region and a drain region.
 18. TheSRAM according to claim 17 wherein said mono-domain region includessubstantially no grain boundary.
 19. The SRAM according to claim 17wherein any grain boundary included in said mono-domain region iselectrically inactive.
 20. An electronic apparatus comprising the SRAMaccording to claim 17, wherein the electronic apparatus is selected fromthe group consisting of a head-mount display, a motor vehiclenavigation, a mobile phone, a video camera, a projector, and a mobilecomputer, wherein the electronic apparatus.
 21. An SRAM comprising: afirst layer comprising at least one first transistor and an insulatingfilm formed over the first transistor; a second layer on the firstinsulating film; wherein the second layer comprises: a pair ofcross-coupled driver transistors; a pair of access transistors; a pairof bit lines electrically connected to the cross-coupled drivertransistors through the access transistors, respectively; and a wordline electrically connected to the pair of access transistors, whereinat least one of the access transistors comprises a crystallinesemiconductor film formed on the insulating film, said crystallinesemiconductor film having a mono-domain region in which a channelformation region is formed, and wherein said crystalline semiconductorfilm comprises a source region and a drain region.
 22. The SRAMaccording to claim 21 wherein said mono-domain region includessubstantially no grain boundary.
 23. The SRAM according to claim 21wherein any grain boundary included in said mono-domain region iselectrically inactive.
 24. An electronic apparatus comprising the SRAMaccording to claim 21, wherein the electronic apparatus is selected fromthe group consisting of a head-mount display, a motor vehiclenavigation, a mobile phone, a video camera, a projector, and a mobilecomputer, wherein the electronic apparatus.